1. Field of the Invention
The present invention relates to a probe coil for a nuclear magnetic resonance apparatus.
2. Description of the Related Art
In general, in an NMR apparatus, there exist a CW type in which a sample is irradiated by an electromagnetic wave of radio frequency signal with a fixed frequency, and a pulse Fourier type in which a sample is irradiated by a pulse-like electromagnetic wave. Recently, however, the latter pulse Fourier type NMR has been often referred to as an NMR apparatus. In the specification in the application, the pulse Fourier type NMR apparatus is to be normally referred to as a nuclear magnetic resonance apparatus.
A fundamental arrangement about an NMR apparatus is described in “Part III, Measuring Technology” in “NMR no sho (The book of NMR)”, by Yoji Arada, Maruzen, 2000. According to the reference, the NMR apparatus is arranged with a superconducting magnet that generates a static magnetic field, a probe that is provided with a probe coil therein for irradiating a sample contained inside with a high frequency pulsed magnetic field and for receiving a free induction decay signal (FID signal) transmitted from the sample, a high frequency power source that supplies a high frequency current to the probe, an amplifier that amplifies the free induction decay signal, a detector that detects the signal, and an analyzing unit that analyzes the signal detected by the detector. About the probe coil, there is a probe provided with a plurality of coils for being applicable to various nuclides and measuring methods. Moreover, the probe coil normally has a function of irradiating a sample with a high frequency pulsed magnetic field together with a function of receiving a free induction decay signal transmitted from the sample.
In a current common NMR, a probe is formed in a saddle type. According to the above-described “The book of NMR”, in the case of the saddle coil, the high frequency pulsed magnetic field, i.e., an RF magnetic field, is directed perpendicularly to the direction of the axis of the coil. Hence, when a static magnetic field is provided to direct perpendicularly, the saddle coil wound around a perpendicularly set sample tube permits measurement with an RF magnetic field applied in the direction perpendicular to that of the static magnetic field.
Meanwhile, there also exists a solenoid type as a form of the probe. In the case of the solenoid coil, an RF magnetic field is directed in parallel with the direction of the axis of the coil. This, for an arrangement in which the solenoid coil is wound around a sample tube, necessitates the static magnetic field to be applied perpendicularly to the direction of the axis of the coil, i.e., in the horizontal direction. According to the above-described “The book of NMR”, in the case of the solenoid coil, because of its easiness in impedance control, a value of a Q-factor, which determines a sharpness in tuning the coil, is enhanced more easily compared with the saddle coil. Furthermore, according to D. I. Hoult and R. E. Richards, “The Signal-to-Noise Ratio of the Nuclear Magnetic Resonance Experiment”, J. Magn. Resonance 24, 71–85 (1976), comparison of probe coils having typical arrangements presented that the solenoid coil has calculated performance about three times higher than that of the saddle coil.
From the foregoing, it has been proved that the form of the probe coil is more preferable in the solenoid type. However, at present, an apparatus in which a vertical static magnetic field is generated by the superconducting magnet is dominant, and most of the NMR apparatus employ the saddle type probe coils. The reason for this is: (1) For employing the solenoid type probe coil, a sample tube and a center bore of the superconducting magnet for generating a static magnetic field must be disposed orthogonal to each other, which necessitates the size of the sample tube to be made small; (2) There is necessity of enlarging the diameter of the center bore of the superconducting magnet, which makes the design of the superconducting magnet difficult; or the like.
Meanwhile, as a measure for improving an S/N ratio, a low-temperature probe (cryoprobe) is sometimes used. According to the above-described “The book of NMR”, the low-temperature probe is referred to as a probe with a system in which a circuit about the probe is made superconducting with the inside of the probe including a preamplifier cooled by low temperature helium gas of the order of 20K. As a superconductor therefor, an oxide superconductor is used.
The low temperature probe has two advantages. One is a capability of enhancing a Q-factor of the coil because of lowered electric resistance of the circuit. The value of Q as the Q-factor of the coil is expressed by the following equation (1):
                    Q        =                                            L              C                                ⁢                      1            R                                              (        1        )            where L is inductance, C is capacitance, and R is resistance. According to the equation (1), it is understood that the Q as the Q-factor is enhanced as the electric resistance R is decreased. The other is improvement in the S/N ratio due to lowered temperature which could reduced thermal noise of the whole circuit. A noise voltage Vn can be expressed by the following equation (2):Vn=√{square root over (4kTΔfR)}  (2)where k is Boltzmann constant, T is temperature, Δf is bandwidth, and R is electric resistance. According to the equation (2), it is understood that the noise voltage Vn becomes small as the temperature T is lowered. Moreover, in common metal, as the temperature T is lowered, the electric resistance R becomes small. Therefore, by cooling the probe to be made superconducting, the noise voltage Vn can be made reduced with a rate more than a rate proportional to R to the one-half power.
Moreover, as a technology relating to the above technology, one is described in Japanese Patent Laid-Open No. 133127/1999 which, for reducing thermal noise at reception, employs a birdcage type probe coil using a superconductor cooled at a low temperature to improve S/N ratio. In this case, high-temperature superconductive material such as YBCO (YBa2Cu3O7−x, yttrium series high-temperature superconductor) is used. The superconductor is applied to only a linear section of the birdcage type coil.
As explained above, by the solenoid type probe coil and the low-temperature probe, there is expected considerable improvement in performance. However, when a low-temperature probe using oxide superconductor is to be applied as a solenoid type probe coil, there arise problems as explained below. (1) An oxide superconductor commonly used in the low-temperature probe is a thin film conductor of YBCO which is difficult to be formed in shapes other than that of a linear conductor with a current technology. In addition, (2) Oxide superconductors including YBCO have a strong relationship between a direction of a magnetic field and a transport current, i.e. have so-called strong magnetic field direction dependence of the transport current, and it is known in the thin film conductor that the critical current is extremely reduced when the magnetic field acts in the direction orthogonal to the film plane. As in the above, it has been difficult up to now to make a probe coil with a complicated form to offer a problem that application to the solenoid type probe coil is difficult. Furthermore, even when using other kinds of superconductors, in a conventional superconductor wire such as a powder-in-tube superconductor wire or a superconductor wire with external stabilizing material formed by conventional extrusion, or a superconductor wire in which a superconductor is formed on a metal substrate of a good electric conductor, the metal provided around the superconductor resulted in functioning as an electromagnetic shield. This has caused a problem of making it impossible to transmit and receive high frequency radio waves.